Although HVDC systems are an idea power generator without inertia, they generate a harmonic wave and consume invalid power. To resolve these problems, a harmonic wave filter, an invalid power compensator such as static VAR (voltage amperes reactive) compensator or STATCOM, and a synchronous phase modifier have been used. However, the invalid power compensator is not associated with the HVDC suitably and a control performance is not secure enough.
FIG. 1 illustrates a conventional synchronous power generator system. A turbine 1-1 controls a frequency and valid power of the synchronous power generator system. An exciter 1-3 controls invalid power and voltages of the synchronous power generator system. The valid power of the synchronous power generator system is expressed by a first area 1-5, and the invalid power of the synchronous generator system is expressed by a second area 1-4.
FIG. 2 is a circuitry diagram showing a conventional hybrid active filter as an example of a power filter for 12-pulse HVDC system. In a passive filter, as time goes by, filter parameters are fluctuated due to the degradation. The passive filter is operatively associated with impedance of a system, which are not known exactly. This causes the stability of the system to be reduced. In contrast to this, because an active filter removes a harmonic wave of the system using a switching element, it has an advantage that may overcome sensitivity to parameters of the passive filter. Further, the hybrid active filter has low impedance at a specific harmonic wave and significantly great impedance of a filter at a fundamental frequency. Accordingly, there is a little electric current inputted from an alternating current power source side. Since the hybrid active filter supplies an electric current to the system at a specific harmonic wave to be removed and a voltage of the harmonic wave is zero, power consumption is small.
FIG. 3 is a view showing a control algorithm of the conventional hybrid active filter shown in FIG. 2, which removes specific harmonic waves such as eleventh and thirteenth harmonic waves. The control algorithm of the hybrid active filter is an algorithm to generate a packing command signal in a phase A switching element of a passive filter.
The hybrid passive filter receives a voltage Va 4-1 measured in a phase A of the system as an input of a Fast Fourier Transform (FFT), and extracts a magnitude V11a and a phase θ11a (4-2) of the eleventh harmonic wave, and a magnitude V13a and a phase θ13a (4-3) of the thirteenth harmonic wave. The control system controls a harmonic wave voltage component to zero by two PI controllers. Outputs of the two PI controllers are changed to amounts of time areas, and the amounts of time areas are added to each other to become a voltage form of f11(t) (4-4). In the same manner, a control system is provided to calculate a voltage command f13(t) (4-6) for removing the thirteenth harmonic wave (4-7). In conclusion, the two voltage commands are added to each other to become a voltage command f(t) (4-5) of the phase A. The same algorithm is used to calculate respective voltage command in phases B and C. In other words, the control algorithm indicated in the hybrid passive filter separates the magnitudes and the phases of the eleventh and thirteen harmonic waves by the FFT, and controls the hybrid passive filter so that the respective separated harmonic components become zero.
FIG. 4 is a circuitry diagram showing a conventional HVDC system having an STATCOM. The conventional HVDC system includes an HVDC 5-1, a STATCOM 5-2, a harmonic filter 5-3, and a capacitor 5-4. The HVDC 5-1 controls valid power. The STATCOM 5-2 compensates and controls invalid power. The harmonic filter 5-3 removes a harmonic wave generated by the HDVC 5-1.
There are three type HVDC systems having an HVDC. A first type HVDC system includes only the HDVC system 5-1 and a STATCOM 5-2. In this type, the STATCOM 5-2 removes a harmonic wave generated by the HDVC 5-1 and compensates and controls an invalid power. A second type HVDC system includes an HVDC 5-1, a STATCOM 5-2, a harmonic filter 5-3, and a capacitor 5-4. In this type, the STATCOM 5-2 controls an invalid power with high precision. Further, the STATCOM 5-2 adsorbs the invalid power at excessive operation as an auxiliary function. A third type HVDC system includes a twelfth filter 5-5 instead of the eleventh and thirteenth filters 5-3 in the second type HVDC system. In the third type HVDC system, the STATCOM functions to compensate detuning of the filter.